1. Field of the Invention
The present invention relates to a zoom lens and, in particular, though not exclusively, to a zoom lens suited for an image pickup apparatus (e.g., a digital still camera and a video camera).
2. Description of the Related Art
For an image pickup apparatus (e.g., a video camera, digital still camera, and other image pickup apparatus as known by one of ordinary skill in the relevant arts and equivalents) using a solid-state image pickup device, a smaller sized zoom lens having a larger zoom ratio would be useful.
A camera of this type includes various optical elements, such as a low-pass filter and a color compensating filter, arranged between a backmost lens and a solid-state image pickup device.
Therefore, a zoom lens used in this kind of camera typically has a relatively long back focus.
A zoom lens that has a leading lens unit with a negative refractive power, which is a so-called negative lead type zoom lens, is an example of an optical lens system that acquires a long back focus.
For a color camera using a solid-state image pickup device for color images, high image-side telecentricity is useful to avoid color shading.
A zoom lens including three lens units in which, from an object side to an image side, a first lens unit has a negative refractive power, a second lens unit has a positive refractive power, and a third lens unit has a positive refractive power are disposed, where the diameter of a front lens element is small, and the image side is telecentic as discussed in, for example, U.S. Pat. Nos. 6,545,819, 6,822,808, 6,862,143, 7,023,625, 6,999,242, and 6,498,687.
A three-unit zoom lens (zoom lens comprising three lens units) of this type in which all lens units move for zooming and chromatic aberration is compensated by using a cemented lens in the second lens unit is discussed in, for example, Japanese Patent Laid-Open No. 2003-005072 and U.S. Pat. No. 6,744,564 and No. 6,809,879.
A three-unit zoom lens, which aims to reduce the number of lens elements, in which a lens element having a negative refractive power in a first lens unit has an aspheric object-side surface and an aspheric image-side surface is discussed in, for example, Japanese Patent Laid-Open No. 2002-055278.
It is useful for a zoom lens used in a video camera or a digital video camera to have a smaller size and a higher zoom ratio.
In the three-unit zoom lens including lens units having negative, positive, and positive refractive powers, respectively, as described above, the stroke of the second lens unit increases as the zoom ratio becomes higher. This results in an increase in the length of the entire lens system, and it is difficult to reduce the size of the zoom lens while achieving a higher zoom ratio. Therefore, many three-unit zoom lenses have a zoom ratio of the order of 3×.
To reduce the thickness of the image pickup apparatus, a so-called collapsible lens system, in which space between lens units can be shortened except during picture taking, is used.
To reduce the thickness of the apparatus, a reduction in the thickness of each of the lens units can be required.
In the three-unit zoom lens described above, the second lens unit is a main lens unit for changing magnification. Therefore, in order to achieve a higher zoom ratio, aberration variations during zooming should be reduced in the second lens unit. To this end, a certain number of lens elements are needed, thus resulting in an increase in the size of the second lens unit.
Consequently, it is important to appropriately set configuration of the second lens unit in the three-unit zoom lens in order to reduce the thickness of the entire lens system while achieving a higher zoom ratio.